A master or information carrier of the above type includes small structures, typically in the nanometer range, which must be produced in a highly evacuated atmosphere. In practice, the energy beam is focused on a recording medium or substrate which is rotated inside a vacuum chamber. By moving the rotating recording medium radially, the energy beam is made to write the information on the recording medium in a helical path, thereby manufacturing a master.
A typical prior art information recording apparatus, known from U.S. Pat. No. 5,446,722, comprises a vacuum chamber, an electron beam generator mounted on a top portion of the vacuum chamber and emitting an electron beam into the vacuum chamber, and a spindle motor driving a spindle to rotate the recording medium in the vacuum chamber. The spindle motor is enclosed in a casing within the vacuum chamber to prevent the magnetic fields of the spindle motor from interfering within the electron beam. The casing is mounted on a movement means effecting radial movement of the rotating spindle. The movement means is in turn mounted on a bottom portion of the vacuum chamber. To reduce friction, the spindle is mounted on a pneumatic bearing on the spindle motor. As a result of the pneumatic bearing, the interior of the casing is in communication with the atmosphere. This prior art apparatus suffers from a number of drawbacks. First, the apparatus is sensitive to changes in ambient temperature and to mechanical vibrations. Such changes will translate into dimensional changes affecting the accuracy with which the structures can be produced on the recording medium. Second, it is a laborious task to mount the electron beam generator precisely perpendicular to the recording medium, complicating both assembly and maintenance of the apparatus. Also, the walls of the vacuum chamber might flex during assembly of the apparatus, thereby changing presumably known dimensional relationships and necessitating excessive fine adjustments on a trial and error basis. Third, it is difficult to completely prevent magnetic fields from entering the vacuum chamber.
A similar information recording apparatus in known from U.S. Pat. No. 5,686,941, in which the radial movement of the spindle is controlled by a feed motor unit arranged outside the vacuum chamber. The feed motor unit drives a screw bar which extends through an opening in a wall portion of the vacuum chamber to a movable holder therein. A spindle motor including a pneumatic bearing is mounted on the holder and is enclosed in a casing communicating with the atmosphere. To seal off the casing from the vacuum chamber, the spindle extends through a magnetic fluid seal which is generated by a magnetic unit producing a magnetic field that interacts with the spindle via a magnetic fluid. Generally speaking, this apparatus suffers from the same drawbacks as the apparatus described by way of introduction. By placing the feed motor unit outside the vacuum chamber, the magnetic field inside the vacuum chamber is reduced, but instead there is an increased risk of leaks, since it is difficult to provide a durable seal between the moving screw bar and the opening in vacuum chamber wall.
Another information recording apparatus having a similar feed motor unit arrangement is disclosed in GB-1,458,778.